A discovery of an alien robot which is more powerful than their own is working on gaining the trust of the Robinsons. But instead of helping them, he plans on sending the family back to his planet of an advanced race of aliens as their slaves.

Recap

Will: What is it?Robot: My enemy is coming.Will: The robotoid?Robot: Negative. I refer to Dr. Smith.Will: You mean he’s the one you’ve been hiding from?Robot: He may change his mind and seek to reassemble me into a pleasure vehicle.Will: I’d never let him do that. Honest I wouldn’t.Robot: If he does, I hope you will ride with me occasionally. I will try very hard not to be substandard.